Magnesium-base alloy



March 6, 1962 G. s. FOERSTER MAGNESIUM-BASE ALLOY Filed Feb. 19, 1960 Il l I l I I l 0 5 Wely/v 2 Per Gen/Rare Ear/6 Me/a/ INVENTOR. George .5Foe/1s rer fiGENT States hce 3,024,108 MAGNESIUM-BASE ALLOY George S.Foerster, Midland, Mich, assignor to The Dow Chemical Company, Midland,Micl1., a corporation of Delaware Filed Feb. 19, 1960, Ser. No. 9,936 8Claims. (Cl. 75l68) This invention relates to magnesium-base alloys. Itmore particularly concerns an improved magnesium-base alloy containingzinc and manganese.

The beneficial effects obtained upon alloying zinc with magnesium arewell known. However, attempts to develop a magnesium-base alloycontaining zinc and manganese which possesses, in rolled form, goodlongitudinal and transverse properties in both directions of rollinghave heretofore been unsuccessful. Magnesium-zincmanganese alloys haveshown poor rollability when the zinc content exceeds about 3 percent.Magnesium-base magnesium-zinc-manganese alloys containing less thanabout 3 percent of zinc have been satisfactorily hot rolled but at thesame time have exhibited poor cold workability, and as the Zinc contentis decreased, the alloys in rolled form possess increasingly poorermechanical properties. The poor properties of conventionalmagnesium-base magnesium-zinc alloys are believed due to heterogenousdeformation which occurs when the alloy is strain hardened as by coldrolling.

It is the principal object of the invention to provide an improvedmagnesium-base alloy containing both zinc and manganese which is readilyrollable and which in rolled form exhibits desirably high tensile andcompressive properties in both the longitudinal and transversedirections of rolling.

Other objects and advantages of the invention will become apparent asthe description of the invention proceeds.

The invention is based on the discovery that in certain limited rangesof proportions of zinc and manganese, herein shown, the addition of rareearth metal to the magnesium-base alloys containing these metals greatlyimproves their properties. In particular it has been found that inmagnesium-base alloys containing from 1.25 to 3 percent of zinc(preferably 1.75 to 2.5 percent), and from 0.1 to 2 percent of manganese(preferably 1 to 2 percent), the addition of a critical amount of rareearth metal, the amount by weight being from 0.04 to 0.2 as much rareearth metal as zinc, the balance being magnesium, a magnesium-base alloyis obtained which in rolled form exhibits good ductility, toughness,formability, resistance to corrosion, and satisfactory weldabilitywithout stress relief and the rolled products have substantially thesame high tensile and compressive strengths in both the longitudinal andtransverse directions of rolling. The invention then consists of theimproved magnesium-base alloy herein described and particularly pointedout in the claims.

The addition of rare earth metals, which ordinarily adversely affectextrudability and room temperature mechanical properties ofmagnesium-zinc alloys in cast form, is decidedly beneficial to themagnesium-zinc alloys in rolled form. Rare earth metal additions improvehot rollability of the high zinc alloys, apparently by decreasing theconcentration of the low-melting magnesium-zinc phase(s) and markedlyimprove cold workability and resultant properties, apparently throughpromotion of more homogeneous deformation. However, the proportion ofrare earth metal added is critical and the addition of too much rareearth metal adversely effects transverse strength, formability, andmechanical properties generally.

A notable feature of the addition of rare earth metal in the criticalconcentration range herein disclosed is the effect on the longitudinaland transverse properties of the alloy in rolled form. The transverseproperties of the said magnesium-zinc-manganese alloys are quitegenerally higher than the longitudinal properties. Yet upon makingsmall, but increasingly larger additions, to a typical example of thesaid Mg-Zn Mn alloy, of rare earth metal covering the said criticalconcentration range, the longitudinal properties of the resultingcompositions in rolled form are found respectively to be successivelylarger While at the same time the transverse properties respectively arefound successively to approach a maximum value and then to decrease tovalues smaller than that of the longitudinal properties. Thecompositions in the range in which the values of the longitudinal andtransverse properties of the alloy approach, become equal, and divergeslightly are those herein disclosed and claimed.

It has not been previously shown that upon varying the proportions of aminor component of an alloy the relative magnitudes of the transverseand longitudinal properties are reversed. Nor is it expected in the artthat such a change is produced by such small changes in the proportionsof a minor component.

As the zinc content of the alloy is increased, the amount of rare earthmetal needed to bring about the transposition of the magnitudes oftransverse and longitudinal properties of the alloy is proportionatelyincreased. To produce an alloy having the desired combination ofproperties, it is therefore necessary to add an amount of rare earthmetal proportionate to the zinc content. Suitable rare earth metal tozinc ratios for the present alloy are those from 0.04 to 0.2 andpreferably from 0.06 to 0.12. These ratios are illustrated graphicallyin the appended drawing.

In the drawing the single figure shows a rectangular coordinate graph inwhich percent zinc is plotted along the ordinate scale and percent rareearth metal is plotted along the abscissa. The range of proportions ofrare earth metal and zinc in the alloy herein disclosed and claimed isgraphically represented by the closed area bounded by the linesconnecting points A, B, C and D, said alloy including from 0.1 to 2percent of manganese and the balance magnesium. The preferred range ofproportions of rare earth metal and zinc is graphically represented inthe same drawing as the closed area bounded by the lines connectingpoints E, F, G and H.

Again referring to the drawing, m agnesium-zinc-manganese-rare earthmetal alloys in rolled form and 1) having rare earth metal-zinc ratioscorresponding to the region generally below the line AD exhibit lowmechanical properties (2) having ratios corresponding to the region tothe right of the line CD generally exhibit low transverse properties,(3) having ratios corresponding to the region above the line BC exhibitpoor rollability and weldability, and (4) having ratios corresponding tothe region to the left of the line AB exhibit low longitudinalproperties and poor rollability.

The rare earth metals suitable for use in preparing the present alloyare: cerium, lanthanum, praseodymium, neodymium or misch metal. Mischmetal with from 35 to percent of cerium, the balance being rare earthmetal and up to 5 percent of non-rare earth metal, is the preferred rareearth metal ingredient of the alloy. Any of the foregoing rare earthmetals may be used alone or in any combination in compounding the alloy.

If desired, thorium may be substituted for all or a portion of the rareearth metal content of the alloy with no loss in properties.Substitution by thorium, however, is on the basis of an equal atomicpercent which corresponds on a weight basis to the use of an amount ofthorium equal to about 1.6 times the weight of the rare earth metalreplaced.

The alloy may be made in the desired proportions according to theinvention by melting together the alloying perature at which crackingwould result if These rolling slabs were scalped to about 1% inchesthickness, heated to about 800 to 850 F. and cross-rolled to a thicknessof about 1 inch, then turned 90 degrees and rolled to about A; inchthickness, and annealed for one hour at 700 F. The annealed strips werethen cold rolled in multiple passes at l to 2 percent per pass to athickness of about 0.1 inch, then annealed for one hour at 700 F. andcold rolled an additional 40 percent. The rolled strip was finally heattreated for one hour at 275 F. The properties set forth in Table 1 weredetermined on the so-prepared rolled strip.

Table 1 Properties of the alloy in rolled form Percent composition, bal.Mg

Alloy Longitudinal Transverse Zn MM Th Mn Per- TYS CYS TS Per- TYS OYSTS cent E cent E Percent E percent elongation. TYS=tensile yieldstrength,

at 0.2 percent deviation from the modulus line.

TS=u1timate tensile strength.

All strengths listed in thousands of p.s.i

MM=miseh metal used consisted of 48% Co, 18% Nd, 5.5%

of other rare earth metals.

to the melt, prior to the settling stage, to improve the malleability ofthe alloy in rolling slab form. An amount of zirconium by weight from0.001 to 0.05 percent of the weight of the alloy is effective. Largeramounts of zirconium are to be avoided as likely to cause precipitationof part of the desired manganese content of the alloy.

In rolling the cast metal, it is desirable first to scalp the cast metalso as to present a smooth clean surface to the rolls of the rollingmill. The clean rolling stock is heated to a suitable rollingtemperature, e.g., about 800 to 850 F. The heated metal is then reducedin thickness by passes between the rolls of the mill, the rolling beingstopped 50 to 100 Fahrenheit degrees short of the temthe rolling werecontinued without reheating the metal. The tern perature to which themetal may decline as it is being rolled before cracking occurs isreadily determined by trial and varies with the proportions of thealloying ingredients. In general in hot rolling, the metal should bereheated when its temperature declines to about 600 or 650 F., ifrolling is to be continued without cracking.

Reductions in thickness of the cast metal of to percent may be made perpass while the metal is at a suitable rolling temperature.

By annealing the hot rolled cast metal, as for example, by heating forone hour at 700 F., the hot rolled metal may be cold rolled (warm rolls,e.g., 180 F.) as much as to 50 percent by making thickness reductions of1 to 2 percent per pass. Before proceeding with the final 40 to 50percent cold reduction by rolling, it is generally desirable to use apreliminary total cold roll of about 15 percent, obtained in multiplepasses, followed by annealing at about 700 F. for one hour to improvethe subsequent cold rollability and mechanical properties.

Examples of the alloy according to the invention were cast in rollingslabs 2 inches by 4 inches by 8 inches.

Pr, and 28.5% La which included minor amounts Compositions of the alloycontaining about 2 percent or more of zinc are desirably worked, as byextruding, before forming into rolled products. Extrudes, of thesecompositions, in the form of slabs make desirable rolling stock andpermit making rolled products by either hot or cold rolling. Forexample, for making rolled products, ingot of the alloy is heated to andextruded at about 600 and 800 F. depending upon the amount of reductionto be made. That is, for high reductions the higher temperatures of therange are used; small reductions may be made at the lower temperaturesof the range. Prior to rolling, the extrude (slab) is heated to betweenabout 800 and 850 F. for example. The heated extrude is rolled in anumber of passes without reheating until the metal requires reheating toavoid cold cracking. On annealing the hot rolled product so obtained,for one hour at about 700 F., for example, it may be cold rolled (warmrolls 180 F.) as much as 45 to 50 percent thinner. The data in Table 2set forth specific examples of the alloy and their properties in theform of pre-extruded rolled strip.

In making this rolled strip, four ingots, each of a different alloy,were cast into three inch diameter billets. These were machined to adiameter of 2 inches, heated to 750 F., then extruded at 700 F. intostrip inch by 2 inches for rolling stock. The extruded strip was heatedto 800 F cross-rolled in several passes to a thickness of inch, turnedand rolled parallel to the direction of extrusion to a thickness ofabout 0.1 inch, reheated, and rerolled at 850 F. in one pass to athickness of A inch. The rolled strip was annealed for one hour at 700F. The annealed strip so obtained was cold rolled about 1 to 2 percentthinner per pass, the total cold reduction being about 40 percent. Thecold rolled strip was heat treated one hour at 275 F., then theproperties set forth in Table 2 were determined.

Table 2 Properties of preextruded rolled alloy Percent composition,balance Mg Alloy No. Longitudinal Transverse Zn MM Mn Perlgcnt TYS CYSTS Pelfient TYS CYS TS Percent E =percent elongation.

TS tensile strength. All strengths listed in thousands of p.s.i.

MM =misch metal used consisted of 48% Ge, 18% Nd, 5.5% Pr, and 28.5% Lawhich included minor amounts of other rare earth metals.

Among the advantages of the invention are that an alloy is providedhaving the light weight characteristic of magnesium and possessingformability, corrosion resistance and good weldability.

This is a continuation-in-part of my copending application Serial No.746,411, filed July 3, 1958, now abandoned.

I claim:

1. A magnesium-base alloy consisting of from 1.25 to 3 percent of zinc,from 0.1 to 2 percent of manganese, and rare earth metal in an amount inpercent by Weight which is from 0.04 to 0.2 of the zinc percentage, thebalance of the alloy being magnesium.

2. The alloy as in claim 1 in which the rare earth metal is misch metal.

3. The alloy as in claim 1 in which up to 100 percent of the rare earthmetal content is rep-laced by thorium, the amount of thorium by weightbeing about 1.6 times the weight of rare earth metal replaced.

4. A magnesium-base alloy consisting of from 1.25 to 3 percent of zinc,from 1 to 2 percent of manganese, and

5. A magnesium-base alloy consisting of 1.75 to 2.5 percent of zinc,from 1 to 2 percent of manganese, and rare rare earth metal in an amountin percent by weight which is from 0.04 to 0.2 of the zinc percentage,the balance of the alloy being magnesium.

earth metal in an amount in percent by weight which is from 0.04 to 0.2of the zinc percentage, the balance of the alloy being magnesium.

6. A magnesium-base alloy consisting of 1.75 to 2.5 percent of zinc,from 1 to 2 percent of manganese, and rare earth metal in an amount inpercent by weight which is from 0.06 to 0.12 of the zinc percentage, thebalance of the alloy being magnesium.

7. The alloy as in claim 6 in which up to percent of the rare earthmetal content is replaced by thorium, the amount of thorium by weightbeing about 1.6 times the weight of rare earth metal replaced.

8. A magnesium-base alloy containing about 2 percent of zinc, 0.15percent of misch metal, and about 1.5 percent of manganese, the balancebeing magnesium.

References Cited in the file of this patent UNITED STATES PATENTS

1. A MAGNESIUM-BASE ALLOY CONSISTING OF FROM 1.25 TO 3 PERCENT OF ZINC,FROM 0.1 TO 2 PERCENT OF MANGANESE, AND RARE EARTH METAL IN AN AMOUNT INPERCENT BY WEIGHT WHICH IS FROM 0.04 TO 0.2 OF THE ZINC PERCENTAGE, THEBALANCE OF THE ALLOY BEING MAGNESIUM.